The present invention relates to a desufurizing composition for molten pig iron. More particularly, it relates to a desulfurizing composition which is suitable for a blow desulfurization in which the desulfurizing composition and a carrier gas are fed into the molten pig iron from a lance nozzle.
The known desulfurizing agents used for the blow desulfurization are, for example, calcium carbides (CaC.sub.2) and calcium carbonates (CaCO.sub.3). Among them, calcium carbide has been considered to be the optimum because of the following reasons.
The desulfurization using calcium carbide (CaC.sub.2) is shown by the following reaction formula: EQU CaC.sub.2 +S.fwdarw.CaS+2C EQU CaC.sub.2 +FeS.fwdarw.CaS+2C+Fe
1 The effect of the desulfurization is remarkable because the desulfurization is the direct reduction of sulfur (S) in the molten pig iron by calcium carbide (CaC.sub.2). PA1 2 The resulting calcium sulfide (CaS) is chemically stable and is not converted into sulfur (S) in the molten pig iron. PA1 3 The reaction is the exothermic reaction and accordingly, the lowering of the temperature in the desulfurization is small.
The other advantageous characteristic of the desulfurization is that calcium carbide (CaC.sub.2) is easily oxidized at high temperature. When oxygen is included in the molten pig iron, calcium carbide (CaC.sub.2) is oxidized to form calcium oxide (CaO) before reducing sulfur whereby the desulfurization function is remarkably decreased. Therefore, it is preferable to use an inert gas such as nitrogen gas as a carrier gas because of stable desulfurization function.
In the desulfurization, the lance nozzle is dipped into the molten pig iron at about 1300.degree.-1400.degree. C. in a depth of about 1 to 2 m. from the surface of the molten iron and the powdery calcium carbide (CaC.sub.2) is fed with the carrier gas of nitrogen etc. into the molten pig iron.
It has been reported that a viscous material is easily adhered on the lance nozzle to clog the lance nozzle or to decrease the area of the opening during the operation. Thus, the shortage or stop of the feeding of the desulfurizing agent is caused and the desulfurization function is remarkably decreased or the desulfurization can not be continued. The adhered material is not easily removed. Thus, the lance nozzle should be often exchanged. This is remarkably uneconomical.
The inventors have studied to overcome these troubles of the clogging of the lance nozzle especially the reason of the trouble in view of said facts. As a result, the fact that calcium cyanamide (CaCN.sub.2) having high molten viscosity is formed by the following reaction of calcium carbide (CaC.sub.2) with nitrogen in the blowing condition and the resulting calcium cyanamide deposits and adheres on the lance nozzle. EQU CaC.sub.2 +N.sub.2 .fwdarw.CaCN.sub.2 +C
The reaction is caused at a relatively low temperature of about 900.degree. C. The production of calcium cyanamide is remarkably increased depending upon the elevation of the temperature. The reaction for producing calcium cyanamide at the desulfurization temperature of about 1300.degree. to 1400.degree. C. is performed at remarkably high velocity.
Thus, it is considered to reduce the reaction for producing calcium cyanamide or to prevent the adhesion of calcium cyanamide on the nozzle even though calcium cyanamide (CaCN.sub.2) is produced, in order to prevent the trouble of the clogging of the nozzle.
The inventors have further studied to develop means for preventing the clogging of the nozzle.